A conventional type of pump jack for pumping oil, water or other liquids from a well comprises a large rocker arm pivotally mounted on a framework. On one limb of this rocker arm is mounted a sucker rod which descends into the well and is connected to the piston of a reciprocatory pump mounted within the well, at the bottom or at some other level from which the liquid is to be pumped. Usually, a counterweight is mounted upon the opposed limb of the rocker arm to counterbalance the greater part of the weight of the sucker rod and piston. To pivot the rocker arm, and thus to reciprocate the sucker rod vertically, the upper end of a crank is fixed to the rocker arm between the counterweight and the pivot. The lower end of this crank is connected to a rotating arm fixedly mounted on a rotating drive shaft positioned below the point of attachment of the crank to the rocker arm. The drive shaft is driven via a gearbox from any conventional type of motor, this motor usually being either an electric motor or an internal combustion engine. The rotation of the drive shaft causes the sucker rod to reciprocate vertically; the motion of the sucker rod is substantially simple harmonic motion, subject only to minor, second-order deviations due to the displacement of the crank from the vertical during the rotation of the drive shaft. Thus, approximately half way through its upstroke the sucker rod is traveling at its maximum velocity and from this point there is applied to the sucker rod a progressively incresing downward acceleration until the sucker rod finally halts at the end of its upstroke. This same downward acceleration is continued into the first part of the downstroke, but decreases progressively until, approximately half way through the downstroke, no acceleration is being applied, althrough the sucker rod is moving downwardly at its maximum velocity. For the remaining half of the downstroke, there is applied to the sucker rod a steadily increasing upward acceleration until the sucker rod reaches the end of its downstroke, whereupon this upward acceleration is continued but a steadily decreasing rate until the upward acceleration ceases approximately half way through the next upstroke. The maximum accelerations imposed upon the sucker rod are considerable; for example, in a typical conventional pump jack having a stroke of three feet and a five second pumping cycle (one upstroke and one downstroke) the maximum acceleration upon the sucker rod is approximately 2.4 feet per second.sup.2.
The loads imposed upon the sucker rod of an oil well pump jack are considerable. During the upstroke in a typical oil well, the weight of the sucker rod and the oil being lifted therewith amounts to about 1.6 pounds per foot of well depth, and thus about 8,000 pounds in a 5,000 foot well (many oil wells are considerably deeper). When a conventional rocker arm oil well pump is in use, it is obvious to even the casual observer that very large shock loadings are being placed upon the sucker rod as the sucker rod reverses its motion at the end of each upward and downward stroke; often the frame supporting the rocker arm can be seen to flex and vibrate, especially as the sucker rod begins its upward stroke. I have concluded that these large shock loadings upon the sucker rod arise because there is a large difference between the upwardly-directed force which is needed to stop the downward stroke of the sucker rod and that necessary to cause the sucker rod to begin its upward stroke. During its downward stroke, the sucker rod and the piston connected thereto do not have to support the weight of the column of oil within the well (obviously, the well is provided with means to prevent the column of oil flowing back down the well as the sucker rod and piston descend). Thus, to stop the downward stroke of the sucker rod, the pump jack need only impose on the sucker rod an upwardly directed force about equal to the weight of the sucker rod and piston. However, during the upward stroke of the sucker rod, not only must the sucker rod and piston be lifted, but also the column of oil within the well. Thus, at the beginning of the upward stroke of the sucker rod, the pump jack must impose upon the sucker rod an upwardly-directed force at least about equal to the weight of the sucker rod, piston and the column of oil in the well. The column of oil in a 5,000 foot well weights above 3,000 pounds and thus at the beginning of each upward stroke this weight is instantaneously imposed upon the sucker rod, resulting in a massive shock loading thereon. Similarly, at the beginning of the downward stroke, the sucker rod is instantaneously relieved of this weight, resulting in another massive shock loading thereon. In a conventional oil well pump jack, no shock-absorbing means are provided to cushion these sudden shock loadings upon the sucker rod, which has no freedom of motion since its position is at all times rigidly fixed by the position of the rotatable arm and the drive shaft. These repeated shock loadings upon the sucker rod tend eventually to cause fractures thereof, leaving a considerable length of broken sucker rod in the well. To retrieve the broken sucker rod, a crew must be employed to fish the broken rod out through the surrounding casing, a procedure which involves considerable expense and a lengthy interruption of production from the well, since pumping of oil therefrom cannot be resumed until the broken sucker rod has been removed and replaced with a new one.
Moreover, althrough this has not previously been realized, I have concluded that the abrupt reversals of sucker rod motion effected by a conventional oil well pump jack are a major cause of the rapid decline in production from an oil well as pumping is carried out over an extended period. It has long been known that when a conventional oil well pump jack is installed in a well, production from the well rapidly falls to a value which is typically about 30 percent of the initial production rate when the pump jack is first installed and thereafter remains substantially constant over an extended period. For example, a typical small Ohio well will produce about 15 barrels of oil per day when the pump is first installed, but within a few weeks production will fall to about four barrels per day and thereafter remain steady for several years. I now believe that one major reason for the rapid decline in production from oil wells is that, when using a conventional oil well pump jack, the sudden reversals of sucker rod motion at the end of the upward and downward strokes cause oscillations and pressure surges (coning effects) in the oil surrounding the pump located at the bottom of the well and these oscillations and pressure surges causes particles suspended in the oil to be forced into the walls of the channels through which oil enters the well thereby clogging these channels and hindering the flow of oil into the well. The pump at the bottom of the well usually comprises a piston attached to the lower end of the sucker rod and reciprocating within a cylinder. At least one check valve is provided at the upper end of the cylinder between the cylinder and a tube which surrounds the sucker rod and through which oil is forced by the piston up to the top of the well. This check valve is open during the upward stroke of the sucker rod to allow oil to flow from the cylinder into the tube but is closed during the downward stroke of the sucker rod in order to prevent unwanted flow of oil back down the tube into the cylinder. The cylinder is provided with at least one perforation through which oil can enter the cylinder, this perforation usually being provided with a check valve which will permit oil flow into the cylinder but not outwardly therefrom. Oil enters the cylinder through this perforation during at least the latter part of the downward stroke of the sucker rod and its attached piston, but cannot enter the cylinder during the upward stroke of the sucker rod.
An oil well takes its oil from a large area surrounding the well, this area usually being of the order of several acres and the oil percolates gradually through the surrounding strata along a multitude of channels toward the oil well. In order to obtain maximum production from the well, it is desirable, I believe, that the flow of oil towards the well be as smooth and continuous as possible and that no sudden pressure surges be allowed to occur within the oil surrounding the well since as explained above, such pressure surges tend to interrupt the flow of oil towards the well and to clog the channels through which oil must percolate.
Unfortunately, the sudden reversals of sucker rod motion produced by a conventional oil well pump jack produce precisely such pressure surges within the well. As already mentioned, a conventional pump jack imposes the maximum acceleration upon the sucker rod at the extremities of its motion. At the beginning of the upward stroke, the large acceleration imposed upon the sucker rod and piston causes an extremely abrupt rise in pressure within the cylinder, a sudden opening of the check valve between the cylinder and the tube, and a very sudden end to the flow of oil into the cylinder, accompanied by a very sudden closure of the check valve or valves which allow oil flow into the cylinder, if these check valves are present. Because a considerable mass of oil is still flowing toward the cylinder, the abrupt cessation of oil flow into the cylinder produces a sudden pressure surge outside the cylinder as the oil "piles up" trying to enter the cylinder and this pressure surge thereafter passes outwardly from the oil surrounding the cylinder into the channels feeding the well, with the undesirable results previously mentioned. Similarly, because of the large downward acceleration imposed upon the sucker rod at the beginning of its downward stroke, a sudden reduction of pressure takes place within the cylinder at the beginning of the downward stroke with a corresponding sudden change in pressure in the oil surrounding the cylinder.
Moreover, it is well known that conventional oil well pumps only pump on each upward stroke a volume of oil equal to a small fraction of the swept volume of the cylinder. As already mentioned, oil enters the cylinder only during some latter part of the downward stroke of the sucker rod and piston and I believe that a major reason for the failure of the cylinder to fill more completely is the comparatively short time which the piston spends near the end of its downward stroke during a conventional pumping cycle. Results of experiments described below indicate that, if the piston is made to spend a longer time traversing the last part of its downward stroke, the resultant inflow of oil into the cylinder on each pumping cycle will be increased by an amount which more than compensates for the resulting increase in pumping cycle time, thus increasing the production of oil from the well.
It will be appreciated that the disadvantages mentioned above are not confined to oil wells, but may be experienced in other wells, such as water wells, which draw liquid from strata surrounding the well and pump it to the surface in substantially the same manner as an oil well.
Accordingly, there is a need for a method and apparatus for pumping a liquid from a well which will avoid the disadvantages of the conventional rocker arm pump jack, and my invention provides such a method and apparatus.